The use of pulses having precisely defined widths is common in many types of electronic circuits. Such pulses can be created with digital logic. It often happens that such a pulse is required to define more than just a width in time; for example, when it is applied to an integrator that is also responsive to the amplitude of that pulse. A common instance of this is when a pulse of precision width and voltage drives an integrator input resistor. Such precision current pulses must have not only a precise duration, but also a precise amplitude, since the total amount of charge applied to the integrator is (for an ideally shaped pulse) a product of the two. Precise current pulses are often used in conjunction with integrators in such applications as DACs (Digital to Analog Converters), which in turn may be used in the precision ADCs (Analog to Digital Converters) that are found in laboratory quality digital multi-meters.
Two trends in current practices have introduced a difficulty in the design of a precision pulsed current source. The first is the trend to place ever more circuitry onto a single IC (Integrated Circuit). While this has many advantages in some respects, it does limit what mix of semiconductor processes is available, and places a premium of keeping power dissipation to a minimum. A second trend in response to the first is that the industry has developed low voltage high speed logic families that operate on 5V or less. It is possible to generate high speed precisely placed pulse edges with these low voltage logic families, but when converted to current pulses the low voltages involved give rise to undesirable reductions in signal to noise ratios. Very often, the designer would prefer to have a much higher voltage to work with, but is compelled to either use the low voltage associated with the newer logic family and suffer the loss in precision, or use an older (and perhaps less available) process with a higher voltage. A third option is to use a modern low voltage high speed logic family for pulse-edge generation, and then apply that to a switching mechanism to shift its level to a suitably high voltage obtained from a stable reference voltage. A number of challenges arise when this latter approach is used in a system that is expected to be extremely accurate, and just how to do it is a significant problem in its own right.